Long-Term In-Orbit Radiation Damage Characterization of SiPMs on GRBAlpha and VZLUSAT-2 CubeSats

Core Concepts

This research paper presents a unique, long-term study of radiation damage in silicon photomultipliers (SiPMs) used in space-based gamma-ray detectors on CubeSats, demonstrating their resilience over three years in orbit and highlighting their potential for future space missions.

Abstract

• Bibliographic Information: Ripa, J. et al. (2024). Characterization of more than three years of in-orbit radiation damage of SiPMs on GRBAlpha and VZLUSAT-2 CubeSats. Nuclear Inst. and Methods in Physics Research, A 00 (2024) 1–13.
• Research Objective: This study aims to characterize the long-term radiation damage effects on SiPMs used in gamma-ray detectors onboard the GRBAlpha and VZLUSAT-2 CubeSats in low Earth orbit (LEO).
• Methodology: The researchers analyzed in-orbit data collected over three years, focusing on the low-energy sensitivity threshold and dark count rate of the SiPMs. They also simulated the expected total ionizing dose (TID) and total non-ionizing dose (TNID) using the ESA's MULASSIS software and Geant4 toolkit.
• Key Findings: The study found that the SiPMs experienced a gradual degradation in their low-energy threshold and an increase in dark count rate over the three-year period. However, the degradation rate appeared to flatten over time, potentially due to a lower proton flux at lower altitudes as the orbital decay progressed and possible long-term annealing effects.
• Main Conclusions: Despite the observed degradation, the study demonstrates that SiPMs, when adequately shielded, can operate effectively in the harsh radiation environment of LEO for extended periods, exceeding three years. This finding highlights the potential of SiPMs for use in future space-based high-energy astrophysics missions.
• Significance: This research provides valuable insights into the long-term performance and radiation hardness of SiPMs in space, contributing to the development of more robust and reliable detectors for future space missions.
• Limitations and Future Research: The study acknowledges the lack of in-orbit temperature-dependent gain calibration measurements as a limitation. Future research could explore this aspect to improve the accuracy of the energy threshold estimation. Further investigation into the observed flattening of the degradation rate, including the potential role of annealing, is also recommended.

Stats

The low-energy threshold of the GRBAlpha detector degraded from ~20 keV to ~70 keV over 3 years and 5 months in LEO.
Simulations estimate an accumulated TID of 230 rad and TNID of 6.5 x 10^6 MeV/g in Si due to trapped protons.
The orbit-averaged integral flux of protons at 550 km altitude is about 60% higher than at 500 km altitude according to the AP-8 model.
The ratio of TID due to trapped protons and electrons (TID(p+)/TID(e-)) ranged from 40-170 for GRBAlpha's orbit.
The ratio of TNID due to trapped protons and electrons (TNID(p+)/TNID(e-)) ranged from 4,100-22,000 for GRBAlpha's orbit.

Quotes

Key Insights Distilled From

Characterization of more than three years of in-orbit radiation damage of SiPMs on GRBAlpha and VZLUSAT-2 CubeSats

by Jaku... at arxiv.org 11-04-2024

https://arxiv.org/pdf/2411.00607.pdf

Deeper Inquiries

How might the findings of this study influence the design and implementation of radiation shielding for future SiPM-based detectors in space missions beyond LEO?

This study underscores the critical importance of radiation shielding for SiPM-based detectors in space missions, even in relatively low radiation environments like LEO. Here's how the findings could influence future designs, particularly for missions beyond LEO where radiation levels are significantly harsher:

Shielding Material Optimization: The study validates the effectiveness of PbSb alloys for shielding SiPMs against radiation damage. Future missions could explore alternative materials or multi-layered shielding incorporating materials with high atomic numbers (for better gamma-ray attenuation) and low atomic numbers (for neutron moderation). This optimization would aim to maximize shielding effectiveness while minimizing weight, a crucial factor in spacecraft design.
Shielding Geometry: The study employed a simple shield design. Future missions could investigate more complex geometries, potentially tailored to the specific radiation environment of the mission. This could involve graded shielding, where the thickness and composition of the shield vary depending on the direction and energy of the incident radiation.
Mission-Specific Shielding: The required shielding will vary significantly depending on the mission's orbit, duration, and scientific objectives. Missions venturing beyond LEO, such as to interplanetary space or the inner solar system, will encounter much higher radiation levels, necessitating more robust shielding. The findings of this study can serve as a valuable baseline for estimating shielding requirements in these harsher environments.
Active Shielding Techniques: Beyond passive shielding, future missions could explore active shielding techniques, such as magnetic fields to deflect charged particles. These techniques could offer enhanced protection, particularly against highly energetic particles found in the outer radiation belts and beyond.
The study's findings provide valuable data points for refining radiation shielding models and simulations, enabling more accurate predictions of SiPM degradation and optimization of shielding designs for future space missions.

Could the observed flattening of the degradation rate be attributed to factors other than lower proton flux and annealing, such as variations in the space environment or intrinsic properties of the SiPMs themselves?

While the study attributes the flattening degradation rate to lower proton flux at lower altitudes and annealing, other factors could contribute to this observation:

Space Environment Variations:

Solar Cycle: The study acknowledges the transition from solar minimum to solar maximum during the observation period. However, more nuanced variations within the solar cycle, such as solar flares or coronal mass ejections, could induce short-term fluctuations in the radiation environment, potentially influencing the degradation rate.
Geomagnetic Activity: Fluctuations in Earth's magnetic field, often driven by solar activity, can alter the trapped radiation belts' shape and intensity. These variations could lead to periods of higher or lower radiation exposure for the CubeSats, impacting the degradation rate.

SiPM Intrinsic Properties:

Manufacturing Variations: Subtle variations in the manufacturing process of SiPMs could lead to slight differences in their radiation hardness. These variations might manifest as differences in degradation rates among the SiPMs used in the study, potentially contributing to the observed flattening.
Radiation Damage Saturation: While not explicitly mentioned in the study, it's possible that the SiPMs are approaching a saturation point in terms of radiation damage. At this point, further exposure to radiation might have a less pronounced effect on their performance, leading to a flattening of the degradation rate.

Other Factors:

Temperature Fluctuations: While the study analyzes temperature dependence, more complex temperature profiles experienced by the detectors throughout their orbits could influence the annealing process and, consequently, the degradation rate.
Data Analysis Methods: The methods used to determine the low-energy threshold and dark count rate, while robust, might introduce small uncertainties that could contribute to the observed flattening.
Further investigation, including more detailed modeling of the space environment and potentially ground-based irradiation experiments on similar SiPMs, would be needed to disentangle these factors and definitively determine their individual contributions to the flattening degradation rate.

What are the broader implications of using miniaturized and increasingly sophisticated detectors like those on CubeSats for advancing our understanding of high-energy astrophysical phenomena?

The use of miniaturized and sophisticated detectors on CubeSats represents a paradigm shift in high-energy astrophysics, offering several significant implications:

Increased Access to Space: CubeSats' low cost and rapid development cycles democratize access to space, enabling more frequent and diverse scientific missions. This accessibility allows researchers to explore a wider range of astrophysical phenomena, including those previously deemed too risky or expensive to study with traditional large-scale missions.
Constellation Missions: The deployment of CubeSat constellations, networks of multiple spacecraft working together, enables simultaneous observations from different vantage points. This capability is particularly valuable for studying transient events like gamma-ray bursts, where coordinated observations are crucial for pinpointing their locations and understanding their evolution.
Targeted Science: CubeSats' small size and agility allow for highly targeted scientific missions. Researchers can focus on specific astrophysical phenomena or energy bands, optimizing the detector design and observation strategies for maximum scientific return.
Rapid Technological Advancement: The drive to miniaturize detectors for CubeSats fosters rapid technological innovation. This leads to the development of more sensitive, compact, and power-efficient detectors, benefiting not only space-based observations but also applications in other fields like medical imaging and particle physics.
Complementing Larger Missions: CubeSats can serve as valuable complements to larger, more expensive space telescopes. They can act as "pathfinders," identifying promising targets for follow-up observations by larger telescopes or providing continuous monitoring of variable sources.
Engaging the Next Generation: CubeSat missions offer unique opportunities for student involvement, fostering the next generation of scientists and engineers. Students can participate in all stages of a mission, from design and development to data analysis and interpretation, gaining invaluable hands-on experience.
The increasing use of miniaturized and sophisticated detectors on CubeSats is revolutionizing high-energy astrophysics, enabling groundbreaking discoveries and expanding our understanding of the universe's most energetic phenomena.